Intruder alarm system



Feb. 7, 1961 H- PEARSON am INTRUDER ALARM SYSTEM Filed May 14, 1957 5 Sheets-Sheet :1

20T-21km Feb- 7, 1961 H. PEARsoN ETAL INTRUDER ALARM SYSTEM 3 Sheets-Sheet 2 Filed May 14, 1957 dOPUmPmD OP Om Wm Feb. 7, 1961 H. PEARsoN ETAL INTRUDER ALARM SYSTEM 3 sheets-sheet s Filed May 14, 1957 H mi-, umili; F

.x a T -la -i d www, c:

95u55 .SU E I L@ 335e L o...

United States Paten-;

2,971,134 INTRUDER ALARM SYSTEM Howard Pearson, New York, N.Y., Vincent T. -McDonough, Verona, NJ., and Theo N. Vassil, Flushing,

i N.Y., assignors to American District Telegraph Cornpany, Jersey City, NJ., a corporation of New Jersey Filed May 14, 1957, ser. No. 659,61

1o claims. (cl. 34a-25s) l The present invention relates to intruder alarm systems, and more particularly to intruder alarm systems of the type in which the approach of an intruder to an area to be protected is registered by an alarm at al remote station.

Intruder alarm systems for protecting outdoor areas have heretofore been suggested `and used. Such systems have taken a variety of forms, including photoelectric systems, capacitance systems and radio frequency sys# tems. Most systems which have been employed can be madesuiciently sensitive so that it is virtuallyimpossible for an intruder to defeat the system, i. `e., enter the protected area undetected. However, `since outdoor protection systems must operate under a variety of weather conditions and since inclement weather creates conditions differing only in degree from the conditions createdby the presence of an intruder, a very high degree Vof syst-enr sensitivity results in an unstable system which'ean-arid often does give a spurious alarm caused by weather con#c ditions. An overly sensitive system can also give spurious alarms as a result of the approach of small `animals or birds. Any protection system which produces a large number of spurious alarms is virtually useless since the cost of operating the system becomes Ainordinately high. As a result, system sensitivitieshave been reduced in an effort to achieve a reasonable balance be` tween sensitivity and stability. e 'l The principal object of the present invention has been' to provide an intruder alarm system which combines irri` proved protection and stability characteristics.

Some protection systems of the prior art have operated with balanced detecting elements' so that weather changes affecting the entire system would not generally produce an alarm. One ditliculty with such a system is" that when, as sometimes occurs, a weather change is effective at one place in the system and not in another, e.g., a slowly advancing wall of rain, the system will go into alarm. Another diiculty with a balanced system of this type is that it is susceptible to defeat by two intruders who approach the detecting elements at spaced points such that the effect of one is balanced by the effect of the other.

An object of the invention has been to provide a novel and improved intruder alarm system which compensates for both general and highly local weather c'h'an'ges and is not susceptible of defect by spaced intruders.

`Other and further objects, features and advantages of the invention will appear' more full'y from the 'following description of the invention. i e e vThe system of the invention is of the type which efr1`- ploys antenna wires disposed about the perimeter of the area to be protected, a transmitting antenna being supplied with a radio frequency signal and a portion of the energy therefrom being picked up by a receiving antenna., In accordance with the invention, the oscillator and re-` ceiver circuits are coupled directly so thatatransfernof energy takes place within the control set and the ,trans-:I mitting and receiving antennas are so disposed in space ICC - plane With the transmitting and receiving wires being spaced from each other but being intermixed so that a substantial mutual capacitance therebetween exists.

The invention will now be described in greater detail with reference to the appended drawings, in which:

Fig. l illustrates a preferred form a antenna array in accordance with the invention;

Fig. 2 is a block diagram illustrating the system of the invention;

Fig. 3 is a circuit diagram of a system in accordance with the invention;

Fig. 4 is an equivalent circuit of the oscillator of Fig. 3;

Fig. 5 is an equivalent circuit of the oscillator and receiver of Fig. 3;

Fig. 6 is anequivalent circuit of the receiver of Fig. 3;

Fig. 7 is a tuning curve for the receiver tuned circuit, showing the effect of adding capacitance to the receiving antenna;

Fig. 8 is a similar tuning curve showing the effect on the receiver circuit of adding capacitance to the transmitting antenna;

Fig. 9 is a schematic illustration of the effect of an intruder on the mutual capacitance between transmitting and receiving antennas;

Fig. l0 is a portion of the transmitter-receiver equivalent circuit illustrating the mutual capacitive coupling produced by an intruder; and "i Fig. l1 shows the effect of a change in mutual capaci` tive coupling on the receiver tuning curve.

Referring now to the drawings, and more particularly to,` Figs. l and 2, there are shown the basic elements of a` protective system in accordance with the invention and comprising an oscillator 20, a receiver 21,. a transmitting antenna array formed by wires 22 and 23 (also designated T), a receiving antenna array formed by wires 24,` 25 and 26 (also designated R), a detector 27, an amplifier 28 and an alarm relay 29.

The oscillator 20, which might be, for example, of the tuned plate type, supplies a suitable radio frequency energy, e.g., 160 kc., to the receiver 21 and to the transmitting antenna array. The transmitting antenna wires 22 and 23 extend along the boundary of the area toV be protected, are parallel to each other, are in the same vertical plane, and are spaced from each other. The receiving antenna wires 24, 25 and 26 are spaced from and lie parallel to and in the same vertical plane as the transmitting antenna wires. Ihe transmitting and receiving wires are spaced from the ground by any suitable distances, such as the 9, 2l, 33, 48 and 72 inch heights shown in Fig; 1. The spacing between adjacent wires y and between the individual wires and ground should be such that the approach of an intruder causes an ap preciable increase in the capacitances to ground of the ed at various points by insulators mounted on suitable poles, as shown, and the wires are preferably maintainedtaut by suitable tensioning means to prevent wind from causing the wires to contact each other or to come close I together. The antenna arrays may be center fed, as'

illustrated, or may be fed from any convenient location,

fammes Y,

ment will be desirable since it may minimizewiringCoStS in multiple installations. The antenna wires may be of any suitable length depending on circuit parameters and other conditions. In practical operation, a length: offabout 300 feet has been found desirable, with a plurality'zvof separate systems operating at different frequencies being used to protect larger areas, although longer lengths can be used. In order to prevent the transmission of radio interference and to make .the mutual capacitance effective as a detecting element, the antenna lengths should be a small fraction of the wave length Aat the operating frequency. For example, at 150 kc., a300 foot array iS only about 1/20 of the wave length. In'some.cas,e's,.'pfar+ ticularly where ground conditions vary widely from, time to time, it will be desirable ,to provide a counterpoise. in the form ofa 'conductive'meshor foil or.' the 'like along the surface of the ground beneath the antennatarraysi This, in effect, provides a 'constant ground'referen'ce. Such a counterpoise may be directly connected to -the electronic chassis ground.'

The antenna configuration necessary to achieve overall weather compensation and weather compensation per unit length and also to prevent defeat by spaced synchronized dual attack is' one in which the transmitting wires T land the receiving wires R are mixed through ,alternate mounting' in a vertical plane. first, third and fifth wires are the receiving antennav while the second and fourth wires are the transmitting antenna For example, as illustrated, the v i an' oscillatorV frequency between Vabout 100 and 200 will be found satisfactory.

A coil 37 is inductively coupled to the coil 32 to provide a direct transfer of energy from the oscillator circapacitance to ground and a capacitor 42 interposed between the inductor v41 and the antenna array R and acting in series with the receiving antenna capacitance to ground. A coupling capacitor 43 is provided to supply the signal developed across the parallel resonant circuit to the double diode 38. This receiving circuit is preferably tuned to a frequency slightly different from the oscillator frequency, the actual difference between the frequencies depending on the Q of the receiving circuit and being selected so that the receiving circuit operates normally on a steep portion of the tuning curve. To pre- -civent detection of stray energy, the coils 32, 33,37 and 41 and deliver energy .to the receiver circuit.` The antenna wires possess both mutual capacitance and direct capaciftance to ground, the capacitance .values vbeing functions of the wire length, wire diameter, Yspacing from .'ground," mutual spacing, and dielectric constant of thev intervening insulation. In this case the dielectric is air and its-iconstant variesslightly about'the value of one with 'changes in atmospheric temperature', pressure and relative 'hu' niidity.

In addition to the capacitive 'coupling' from wire to wire and from wire to ground, the antenna-wires 'are linked together with an electromagnetic field generated by the antenna wires T. This field has two components,l an induction field and a radiation field. f The induction field is'. relatively strong near the wiresTbut. attenuates rapidly as the cube of the`distancefrom the wires. l The` radiation field is .relatively weak near the vwires T. and attenuates slowly in direct proportion tothe distance from'. the wires. The reactive .induction field transfersener'gy from the antenna wires'T to the receiver 'wiresfR= by'the electromagnetic coupling of these V.wires in air. vThis re# ceived energy isj superimposed, in the' receiver.' unit 121," on similar energy directly transferred fromV the oscillator 20.

tothe receiver 21.v The polarity of thev c'onnecti'onsitoV anode c1rcu1t of tube 49. The other Vcoil 52B of the coil 37 must be as shown in Fig. 3 to provide maximum sensitivity for the system. Rapid changes in the level ofthe combined signals constitute an alarm voltage indicati ing an attempt to enter upon the protected premises; Such rapid changes will occur when the approach of `an intruder increases the mutual capacitive coupling'be'tween thewiresTandR.V i Referring now to Fig. 3, oscillator tube 30 may conf' veniently be included in av tuned plate oscillator circuit:` The anode of tube.30 is supplied witha suitable DIC; potential froma power supplyl31 through a coil 32. Re-Q generative feedback is providedby av coil`33 coupled't,v the Coil 32 and includedin the control .grid circuit ofk tube, 30. vThe anode of4 tube 30is coupled to-theftr'ffansf'4 mitting antenna array-T through a vcapacitor34 and a' capacitor 35, connected in series.- The frequency determining elements of the oscillator circuit are the coil 32 and the series capacitances of capacitors 34, 35 and *theV antenna wires T to ground. Tuning may be-effectel by varying the inductance of coil 32 orby making one ory I' more of lthe capacitors variable'. Y Forl most purposes? andthe detector circuit are preferably shielded, as shown. The composite A.C. signal delivered to the tube 38 (-directly from the oscillator circuit and by way of antenna coupling) is rectified and doubled, resulting in a D.C. voltage between terminal 44 and ground.` Undernormal conditions, this D.C. voltage will not change and hence no signal voltage will be applied to the Qontrol grid of a pentode amplifier tubev 46 through `a coupling capacitor47 and a potentiometer 48. Capaci-l tor 47 yand potentiometer 48 form a differentiating circuit with a time constant such that slow changes in the D.C.lvolt age, suchas are caused by weather changes, will not appear at the grid of tube 46. A time constant ofthe order of 20 seconds has been found desirable for7 "i most conditions, although other timevconstants maybe.

used.'l Y The time constant should not be too small or defeat of the system by an extremely slow moving-intruderwill be possible. More rapid changes in the D C. voltage between terminal 44 ,and ground, such as are capsedmbyjanj Vintruder approaching the protected perimeter, will be transmitted by capacitor 47 and will be amplified by tube 46, Theamplified output 0f tube 46 is suppliedto the control grid'of a second-oentode am plitier tube 49 through a coupling capacit ,-r 50. The capacitor 50, ltogether with a resistor 51, forms another differentiating circuitiwhich preferablyhas a timev constantof about 20 seconds, further insuring that slow changesjingDLCf.voltage` level will not yield an alarm. A` coilSZA of a differential relay is included in the differential relay is supplied with a constant current nfrom' the power supply 31 through a potentiometer 53 and a resistor 54. The differential relay is held in an unope'rated position bybalanced current flowing in windings 52A 4 and 52B ,jwhich 'isth'esituation which will exist in the absence of a signal voltage at the control grid of tube 49. The presence of such a signal voltage, which will occur whenan 'attemptis being made Vto cross or approach the protected perimeter, will change the current flowing through coil 52A, causing the differential `relay t o operate, in turn resultinggin the'ttransmission of an alarm to'agremote station. -The change in current neces.` sary to operate the differential relay and hence to trans'- rnit an alarm-is-preferably small. System sensitivity can easily be adjusted by changing the'setting of potentiometerl48 which acts as a gain control. p

z Acapacitor 5S,v which intercouples the input and out-j put circuits of tube 46, may be provided. to give negative feedback forsteepfront vpulses such as are produced by lightning.. pulses. i

'and' reducesthe amplification Y of afavnr'aa determined by the value of parallel connected capacitor 57 and resistor 58. Such tiashing of the neon lamp 56 will result in the amplification of voltage pulses by tubes 46 and 49 and the operation of the diterential alarm relay. Similarly, failure of tube 46 through low emislsion will cause a neon lamp 59 to Hash at a rate determined by parallel connected capacitor 60 and resistor 61,

-again transmitting an alarm. Failure of tube 49 will directly cause the alarm relay to operate Ithrough loss of current in winding 52A. f Further circuits between the alarm contacts of the diiferential relay and the remote station are also preferably supervised. For example, means should be provided, as is customary, to transmit an alarm upon tampering with the control set or upon a power failure.

While the illustrated antenna construction of ve wires is preferred, four wires or a greater number of Wires can be used. When four wires are used, mutual capacitance may be increased to provide better operation by provision of a capacitor 62 joining the transmitting and receiving antennas. Such a capacitor might have a value, for example, of 300 micro-microfarads.

The operation of the system of the invention will be better understood by reference to certain equivalent circuits now to be described. Fig. 4 is the equivalent circuit of the tuned plate oscillator, L being the inductance of the oscillator coil 32, ICo being the total capacitance of series capacitors 34, 35 and the capacitance to ground of the .transmitting antenna, plus any capacitance which may be added for tuning. The resistor ro represents the circuit resistance including the resistance of coil 32, the ground resistance, etc.; rp is the dynamic plate resistance of tube 30; ip` is the oscillator plate current; if, is the oscillator tank circuit current; ,u. is the ampliiication factor of tube 30; and eg is the voltage induced into the grid circuit of tube 30 through the inductive coupling of coils 32 and 33.

Under proper circuit conditions, oscillations will be sustained between the grid circuit and the tank circuit with the frequency of oscillation fo given by the following equation:

f 1 [ro-|- TI, o 21m/LOC., TD

Because of the small energy radiation, the value of ro is small compared with the value of rp, so that for practical purposes the oscillator frequency can be expressed as 1 "marmo,

The equivalent circuit for the receiver and oscillator is shown in Fig. 5, where L represents the inductance of coil 37, Lr represents the inductance of coil 41, Cr represents the capacitance of capacitor 42 in series with the capacitance to ground of the receiving antenna together with any loading capacitance which may be added in tuning the system. The resistor rr represents the resistance of the coils 37 and 41 together with the receiver ground resistance, etc. M represents the mutual inductance of coils 32 and 37 and 1'r is the receiver circuit current. The resonant. frequency fr of the receiver circuit is given by the equation:

Where-LQ. is the combined inductance of L and 1.,.

The receiver equivalent circuit alone is shown in Fig. '56 where er is the voltage induced in the receiver ycircuit di, dt

being the time rate of changegof the oscillator tank current in. The voltage ed of Fig. 6 is the detector input voltage which is supplied to the detector 38 through capacitor 43. The dotted capacitors C, and Cir shown in Figs. 5 and 6 represent the added capacity presented by an intruder approaching or attempting to pass between the antenna wires, C, being the added transmitting antenna capacity to ground and (2 being the added receiving antenna capacity to ground.

In adjusting the system, the value of lLr (coil 41) is varied to obtain a peak value of current. ir (in the absence of Cir) and consequently a peak voltage across C, is achieved. This voltage is equal to i,XCr where XCr is the capacitive reactance:

At this point the resonant frequency of the receiver circuit is equal to the oscillator frequency and the inductive reactance is equal to and out of phase with the capacitive reactance.

The receiver circuit is then tuned ot resonance by adjustment of coil 41 to an intermediate point on the tuning curve, for example, the point O in Fig. 7.

The voltage ed at the receiver output is rectied and doubled by the detector stage tube 38, and changes in ed are amplified in two stages (tubes 46 and 49), and control the current through differential relay winding 52A, changes in this current resulting in the transmission of an alarm signal to the central station or other alarm receiving headquarters.

Approach of an intruder to the antenna system results in a net change in alarm relay current which is the combined net result of three distinct effects:

(1) Increase of the capacitance of the receiver wires R to ground, causing a change in alarm current in one direction;

(2) Increase of the capacitance of the transmitting wires T to ground, causing a change in alarm current in the opposite direction; and

(3) A change in the4 mutual capacitive coupling between the receiving and transmitting wires resulting in the transfer of a Voltage from the oscillator circuit into the receiver circuit in opposition to the steady state voltage of the receiver circuit; this condition tending to increase the alarm current through winding 52A irrespective of whether the receiver circuit is tuned on the high or low frequency side of the tuning curve.

Considering the first effect and referring particularly to Figs. 6 and 7, the addition of intruder capacitance C1, causes a drop in the voltage ed from O to O because of the drop in ir caused by the shift in the tuning curve from and the drop in the capacitive reactance from 1 Considering next the second effect and referring particularly to Figs. 5 and 8, addition of Vintruder capacitance C1 to the oscillator circuit reduces the generated frequency 1 A i f--.- f Y 21m/L00,

by D, -to ,Y

l fin= 1 y 21u/Low# Ca This shiftsthe operating point (Fig. 8) from O to O" andfraises the voltage ed. With the receiver circuit tuned to a frequency lower than the oscillator frequency, the voltage ed tends to drop due to the capacitance added tothe receiving antenna by the intruder, while the voltage ed' tends to increase due to the capacitance added to the transmitting -antenna by the intruder. In practice, these eiects on the voltage ed substantially cancel each other. With the receiver circuit tuned on the high side of the oscillator frequency, the eiects of intruder capacitance on the transmitting circuit and receiving circuit will be reversed, as will be apparent from reference to Figs. 7 and 8.

Considering next the eiect on the voltage ed produced by a change in mutual capacitive coupling between the transmitting and receiving wires and referring particularly to Figs. 9, l and 11, it Will be observed that the added capacitive coupling Cc produced by an intruder sets up a voltage in the receiver circuit which is in opposition to the steady state voltage set up therein by the mutual inductive coupling M. The steady state mutual capacitive coupling between the wires is small and the voltage'transferred thereby to the receiver circuit, while in opposition to the inductively transferred voltage',will be small in comparison with the latter. The manner in which an intruder increases the mutual capacitive coupling is illustrated in Fig. 9 where I represents the intruder and the capacitors represent the capacitances between the respective wires R and T and the intruder I.

` Presence of the opposition voltage in the receiver circuit caused by the mutual capacitive coupling Cc causes a drop in the peak voltage of the resonance curve X (Fig. 1l), in eifect producing a new curve Y. The voltage ed will thus drop from O to 0. This change in voltage ed will be a decrease irrespective of whether the receiver circuit is tuned above or below the oscillator frequency. The voltage change OO (Fig. 7) will be nearly equal and opposite to the voltage change O-O" (Fig.- 8) so that the voltage drop O-O" (Pig. 11) will be the controlling voltage which causes an increase in alarm current because of the approach of an intruder.

When weather changes occur the capacitance to ground of the antenna wires will change, but the eifects will substantially balance out, as discussed in connection with an intruder. Any net change in voltage ed resulting therefrom and any change in voltage ed resulting from the slight change in mutual capacitance that may be caused by weather, will occur. over a relatively long period of time and hence will be prevented from producing any material effect on the alarm current by the delay circuits 47--48 and StL-51, as described. While any outdoor protection system using intruder capacitance as a detecting element will be affected to a greater or lesser degree by weather changes, the 'system of the invention exhibits rgreater stabilitythan those of the prior art and, when the system is properly adjusted, can only be caused to give a spuriousU alarm upon an extremely rapid and severe weather change. In the systems of the 8 prior art in which an antenna extending in one direction is balanced against an antenna extending in the other direction, a local weather change affecting one antenna and not the other, such as an advancing wall of rain, often produces a spurious alarm because of the result ing imbalance. Such an imbalance cannot occur in the system of the invention since the receiving and transmitting antenna arrays are equally exposed to such local variations. p

While the invention has been described in connection with a specific embodiment thereof and in a specific use, various modifications thereof will occur to those skilled in the art without departing from the spirit and scope of the.invention, as set forth in the appended claims.

What is claimed is:

1. An intruder alarm system for providing an alarm signal indication of the approach by an intruder to a boundary of an area to be protected, comprising a transmitting antenna array formed fromV a plurality of spaced conductors disposed generally in a vertical plane and ex tending along said boundary of said area, a receiving antenna array formed from a second plurality of spaced conductors disposed generally in said vertical plane and extending along said boundary of said area, the conductors of said transmitting and receiving antenna arrays being generally parallel to said boundary and being intermixed by alternate spacing in said vertical plane, the spacing between adjacent conductors and between individual conductors and ground being selected so that appreciable increases in antenna capacitance to ground and in mutuall capacitance between the conductors of `said transmitting antenna array and the conductors of said receiving antenna array occur upon the approach of an intruder to said boundary, a variable source of radio frequency signals operating at a given frequency, means to supply a portion of the signal energy from said source to said transmitting antenna array, a receiving circuit coupled to said receiving antenna array whereby a portion of the signal energy supplied to said transmitting antenna array appears in said receiving circuit through an air-borne coupling between said antennas,V said receiving circuit, in combination with said receiving antenna array, being tuned to a frequency in the neighborhood of said given frequency, circuit means to supply another portion of the energy from said source directly to said receiving circuit, and means responsive to changes in the average signal level in said receiving circuit occurring faster than a predetermined rate to provide said alarm signal indication.v

2. An intruder alarm system for providing an alarm signal indication of the approachr by an intruder to a boundary of an area to be protected, comprising a transmitting antenna array formed from a plurality of spaced conductors disposed generally in a vertical plane and extending along said boundary of said area, a receiving antenna array formed from a second plurality of spaced conductors disposed generally in said vertical plane and extending along said boundary of said area, the conductors of said transmitting and receiving antenna arrays being generally parallel to said boundary and being intermixed by alternate spacing in said vertical plane, the spacing between adjacent conductors and between individual conductors and ground being selected so that appreciable increases in antenna capacitance to ground and in mutual capacitance between the conductors of said transmitting antenna array and the conductors of said receiving antenna array occur upon the approach of an intruder to said boundary, a variable source of radio fre quency signals operating at a given frequency, means to supply a portion of the signal energy from said source to said transmitting antenna array, a receiving circuit coupled to said receiving antenna array whereby a portion of the signal energy supplied to said transmitting antenna array appears in said receiving circuit through an air-,bornecoupling between said antennas, saidrey boundary of an area to be protected, comprising a transmitting antenna array formed from' a plurality of spaced conductors disposed generally in a vertical plane and extending along said boundary of said area, a receiving VIl! antenna array formed from a second plurality of spaced conductors disposed generally in Asaid vertical plane and extending along said boundary of said area, the conductors of said transmitting and receiving antenna arrays being generally parallel to said boundary and being int'ermixed by alternate spacing in said vertical` plane, the spacing between adjacent conductors and between indi- `vidual conductors and ground being selected so that ap-` preciable increases in antenna capacitance to ground and in mutual capacitance between the conductors of said transmitting antenna array and the conductors of said receiving antenna array occur uporrthe approach of an intruder to said boundary, a variable source of `radio frequency signals operating at a given frequency, means to supply a portion of the signal energy from said source to said transmitting antenna array, a receiving circuit coupled to said receiving antenna array whereby a portion of the signal energy supplied to said transmitting antenna array appears in said receiving circuit through an airborne coupling between said antennas, said receiving circuit, in combination with said receiving antenna array,`

being tuned to a frequency slightly different than said given Ifrequency, circuit means to supply another portion of the energy from said source directly to said receiving circuit, said directly supplied energy and said energy derived from said transmitting antenna array being in phase opposition in said receiving circuit, a detector circuit coupled to said receiving circuit to provide a direct current voltage proportional-to the average signal level in said receiving circuit, and means responsive to i,

changes in said direct current voltage occurring faster than a predetermined rate to provide said alarm signal indication. v

' 4i An intruder alarm system for providing an alarm s`ign`a1=i`ndication of the approach byH an intruder to a boundary of an area to be protected, comprising a transmitting antenna array formed from a plurality of spaced conductors disposed generally in a vertical plane and extending along said boundary of said area, a receiving antenna array formed from a second plurality of spaced conductors disposed generally in said vertical plane and extending along said boundary of said area, one of said pluralities of conductors consisting of two conductors and the other plurality of conductors consisting of three conductors, the conductors of said transmitting and receiving antenna arrays being generally parallel to said boundary and being intermixed by alternate spacing in said vertical plane, the spacing between adjacent conductors and between individual conductors and ground being selected so that appreciable increases in antenna capacitance to ground and in mutual capacitance between the conductors of said transmitting antenna array and the conductors of said receiving antenna array occur upon the approach of an intruder to said boundary, a variable source of radio frequency signals operating at a given frequency, means to supply a portion of the signal energy from said source to said transmitting antenna array, a receiving circuit coupled to said receiving antenna array whereby a portion of the signal energy supplied to said transmitting antenna array appears in said receiving circuit through an air-borne coupling between said antennas, said receiving circuit, in combination with 10 said receiving antenna array, being tuned to a frec'fuentty slightly different than said given frequency, circuit means to supply another portion of the energy from said source directly to said receiving circuit, and means responsive to changes in the average signal level in said receiving circuit occurring faster than a predetermined rate to provide said alarm signal indication.

5. An intruder alarm system, as set forth in claim 4, in which the conductor located closest to the ground is not more than about 9 inches from the ground and in which the conductor located furthest from the ground is not less than about 72 inches from the ground.

6. An intruder alarm system, as set forth in claim 4; in which the conductor located closest to the ground, the conductor located furthest from the ground, and the conductor located nearest to the middle between said closest and furthest conductors form said receiving antenna array, and in which the other two conductors form said trans-v mitting antenna array.

7. An intruder alarm system for providing an alarm signal indication of the approach by an intruder to a boundary of an area to be protected, comprising a transmitting antenna array formed from a plurality of spaced conductors disposed generally in a vertical plane and extending along said boundary of said area, a receiving antenna array formed from a second plurality of spaced conductors disposed generally in said vertical plane and extending along said boundary of said area, the conductors of said transmitting and receiving antenna arrays be-j ing generally parallel to said boundary and being in termixed by alternate spacing in said vertical plane, the spacing between adjacent conductors and lbetween indi# vidual conductors and ground being selected so that ap-` preciable increases in antenna capacitance to ground and in mutual capacitance between the conductors of said transmitting antenna array and the conductors of said receiving antenna array occur upon the approach Vof an intruder to said boundary, a capacitive element inter` coupling said antenna arrays to augment said mutual capacitive coupling therebetween, a variable source of radio frequency signals operating at a given frequency,-

means to supply a portion of the signal energy from said source to said transmitting antenna array, a receiving circuit coupled to said receiving antenna array wiheref by a portion of the signal energy supplied to said transy mitting antenna array appears in said receiving circuit through an air-borne coupling between said antennas, said receiving circuit, in combination with said receiving' antenna array, being tuned to a vfrequency `slightly different than said given frequency, circuit means to supply another portion of the energy from said source directly to said receiving circuit, and means responsive to changes in the average signal level in said receiving circuit occurring faster than a predetermined rate to provide said alarm signal indication.

8i An intruder alarm system for providing an alarm signal indication of the approach by an intruder to a boundary of an area to be protected, comprising a transmitting antenna array formed from a plurality of spaced conductors disposed generally in a vertical plane and extending along said boundary of said area, a receiving antenna array formed from a second plurality of spaced conductors disposed generally in said vertical plane and extending along said boundary of said area, the conductors of said transmitting and receiving antenna arrays being generally parallel to said boundary and being intermixed by alternate spacing in said vertical plane, the spacing between adjacent conductors and between individual conductors and ground being selected so that appreciable increases in antenna capaictance to ground and in mutual capacitance between the conductors of said transmitting antenna array and the conductors of said receiving antenna array occur upon the approach of an intruder to said boundary, a variable source f radio frequency signals operating at a given frequency, said source being an oscillator having a tank circuit iucluding. the capacitance to ground of said. transmitting antenna ,array whereby a portion of the signal energy 'from' said source is supplied to said transmitting antenna array," a receiving circuit coupled to said receiving antenna array whereby a portion of the signal energy supplied to said'transmitting antenna array appears in said receiving circuit through an air-borne coupling between said antennas, said receiving circuit, in combination with said receiving antenna array, being tuned to a frequency slightly different than said given frequency, inductive coupling circuit means to supply another portion of the energy from said source directly to said receiving circuit, means responsive to changes in the average signal level in said receiving circuit to provide said alarm signal indication, and means to suppress formation of said alarm signal indication for changes in said average signal level occurring ,slower than at a predetermined rate.

"9"."An intruder alarm system for providing an alarm signal indication of the approach by an intruder to a boundary of an area to be protected, comprising a transmitting antenna array formed from a plurality of spaced conductors disposed generally in a vertical plane and extending along said boundary of said area, a receiving antenna array formed from a second plurality of spaced conductors disposed generally in said vertical plane and extending alongsaid boundary of said area, the conductors of said transmitting and receiving antenna arrays being generally parallel to said boundary and being in-A termixed by alternate spacing insaid vertical plane, the spacing between adjacent conductors and between individual conductors and ground being selected so that appreciable increases in antenna capacitance to ground and in' 'rnutual capacitance between the conductors of said transmitting antenna array and the conductors of said receiving antenna array occur upon the approach of an intruder to said boundary, a variable source of radio frequency signals operating at a given frequency lying in the range of about 100-200 kc., the length of said boundary being a small fraction of the wave length of said given frequency to prevent the transmission of radio interference, means to supply a portion of the signal en ergy from said source to said transmitting antenna array, a receiving circuit coupled to said receiving antenna array whereby a portion of the signal energy supplied to said transmitting antenna array appears in said receiving circuit through an air-borne coupling between said antennas, said lreceiving circuit, in combination with said receiving antenna array, being tuned to a frequency slightly-diferen't than said given frequency, the ldifference between thetunedfrequency of said receiving circuit-.and said given frequency being selected to provide operation on a steep portion of the tuning curve of said'receving circuit, circuit means to supply another portion of the venergy, from said source directly to said receiving circuit, and meansfresponsive to changes `in the average signal level in said receiving circuit occurringfastei than a predetermined rate to provide said alarm signal indication. 10. An intruder alarm system for providing an alarm signal, indication of the approach by anintruder to a boundary'of an area to be protected, comprising a transmitting antenna array formed from a plurality of generally parallel conductorsspaced from each other. and from the ground and extending along said boundary of said area, a receiving antenna varray formed from a second plurality of generally parallel conductors spaced from each other and from the ground and extending along said boundary of said area, the conductors of said transmitting and receiving antenna arrays being generally parallel to Said boundary and being intermixed by alternate spacing, the spacing between adjacent conductors and between individual conductors -and groundbeing selected so that appreciable increases in antenna capacitance to ground and in mutual capacitance between the conductors of said transmitting antenna array and the conductors of said receiving antenna array occur upon the approach of an intruder to said boundary, a variable source of radio frequency signals operating at a given frequency, means to supply a portion ofthe signal energy from said source to said transmitting antenna array, a receiving circuit cou-A pled to `said receiving antenna' array whereby a portion of the signal energy supplied to said transmitting anten-` na array appears in said receiving circuit through van airborne coupling between Asaidantennas, said receivingcircuit, in combination with said receiving antenna array, being tuned to a frequency in the neighborhood of said given frequency, circuit means to supply another portion of theenergy from said source directly to said ,receiving circuit, and means responsive to changes in the aver- 'age signal level in saidreceiving circuit occurring faster than a predetermined rate to provide said alarm signal indication.

References Cited in the tile of this patent l UNITED STATES PATENTS 

